Using Regex Find/Change to change by Unicode

I often have to handle diacrit characters in a non-unicode (8-bit) fonts, which need changing to their unicode equivalent.

To do this I can use a Grep search in Indesign's Find/Change to, for example, replace character \x{00E1} with character \x{0101} for ā

It's not a big issue for me, but wondered why this is not possible with the Regex Find/Replace in Sigil.

I looked in the documentation but could not find any information about this there.

Thanks
Jim